The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the imaging of soft tissues and implanted devices such as brachytherapy seeds.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles under the direction of prescribed pulse sequences in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
An advantage of MR imaging over other imaging modalities such as x-ray CT is its ability to differentiate between different soft tissues. The amplitude of the received NMR signal during a measured cycle is determined by a number of factors related to the molecular bonding of the excited spins with surrounding atoms. These factors can be magnified, or xe2x80x9cweightedxe2x80x9d by the choice of imaging pulse sequence scan parameters to enhance the contrast between different tissue types.
Unfortunately, MR imaging does not provide optimal contrast between certain implants and surrounding soft tissues. Seeds used in brachytherapy of the prostate, for example, are small cylindrical-shaped objects made of titanium which are implanted in a malignancy. Accurate placement of these seeds is critical for the proper treatment of the disease and it is normal practice to image the prostate following implantation to confirm placement. Currently, x-ray CT is the preferred imaging modality for this purpose because it produces an image in which the seeds are highly contrasted with surrounding soft tissues. The limitation with x-ray CT, however, is that it does not contrast one soft tissue from another very well, making it difficult to discern the prostate tissue and the malignancy therein.
The present invention is a method for acquiring and reconstructing an MR image which provides the high contrast between different tissues normally associated with this imaging modality and high contrast between implanted objects and surrounding soft tissues. More particularly, a complex k-space data set is acquired with the MRI system; a magnitude image is reconstructed from the k-space data set; a phase image is reconstructed from the k-space data set; the location of the implanted objects is determined from the phase image; and the determined location of the implanted objects is indicated on the magnitude image.
An object of the invention is to produce an image which differentiates between soft tissues and differentiates between implanted objects and surrounding soft tissues. It has been discovered that a phase MR image provides sharp contrast between implants and surrounding soft tissue. When combined with a conventional magnitude image that differentiates between soft tissues, the phase image provides the additional contrast mechanism for locating the implants.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.